Solid state light source based lamps (including light-emitting diode (LED) based lamps) and gas discharge lamps, such as high intensity discharge (HID) and fluorescent lamps, may be used in connection with a wide variety of applications. Such lamps are typically driven by a ballast circuit. The ballast circuit typically converts an input signal to a stable direct current (DC) or alternating current (AC) voltage used to drive the lamp. The ballast circuit may, for example, incorporate a rectifier to receive an AC input and a power conversion circuit. The power conversion circuit may receive an unregulated output from the rectifier and provide a stable, regulated output to the lamp.
When it is desired to provide an adjustable output illumination level for a lamp, a dimming control circuit may be used. The dimming control circuit may receive line voltage, e.g. from a 120 VAC/60 Hz source, and provide a modified output signal to the ballast rectifier for the purpose of controlling the illumination level of the lamp. In one configuration, the dimming control circuit may be a circuit known as a “phase control” dimmer or a “phase-cut” dimmer.
In a phase control dimmer, a fraction of the input voltage sine-wave is cut in each period of the waveform, i.e. the conduction angle of the input voltage sine-wave modified. During the cut-time interval or “dead time” when the voltage is cut, the output of the phase control dimmer may be substantially zero. The residual time interval where the voltage differs from zero is known as the “dimmer conduction time.” Both the dimmer conduction time and the dead time are variable, but the time period of the input voltage waveform is constant, e.g. 1/60 second in the United States. As used herein, the “dimmer setting” refers to the ratio of the dimmer conduction time to the time period of the input waveform. The dimmer setting of a phase control dimmer is controllable by a user. In one configuration, the dimmer setting may be varied from about 0.78 to about 0.25.
The ballast circuit may be configured regulate the lamp light output in response to the dimmer setting. In one configuration, the ballast circuit may include a conduction angle detection circuit for providing an output representative of the dimmer setting to the ballast power conversion circuit. The power conversion circuit may be configured to drive the lamp for establishing a lamp light output corresponding to the output of the conduction angle detection circuit.
One known phase-control dimming setting detection circuit is a simple RC filter. The rectified output of the dimming control circuit may be provided to the input of the RC filter, and the output of the filter may be a DC signal proportional to the dimmer setting signal. Although an RC filter configuration is simple, it may provide an output that is sensitive not only to the dimmer setting but also to fluctuations in the line source voltage amplitude, which may result in undesired changes in the light output at a constant dimmer setting. Also, an RC filter configuration may not provide sufficient linearity of lamp light output regulation. Another known conduction angle detection circuit incorporates a microcontroller, which adds complexity and cost to the ballast circuit.